Remove die() and exit()

Summary: These functions do still exist but not in HNI anymore as they were only used by the following syntax:

```php
$x = "exit";
$x();
```

Part of #3525.
Closes https://github.com/facebook/hhvm/pull/3552

Reviewed By: @sgolemon, @ptarjan

Differential Revision: D1512234
VASM part 7: use a Vreg temp in ArgGroup, finish removing m_rScratch

Summary: Finish converting remaining uses of m_rScratch to virtual (temp)
registers, and extend ArgGroup to allow srcs to be Vreg temps.
refactor shuffle2 into a copyTV helper functions.

Reviewed By: @alexmalyshev

Differential Revision: D1460426